Brain Introduction

How it Works

Like other parts of your body, your brain is made up of cells. Many of these cells help regulate the chemistry of the brain and give it structure. But some are specialized to do far more. These cells, known as neurons, do most of the work that allows you to think, feel, and move.

A human brain may contain as many as 100 billion neurons of different kinds. The neurons connect through long, spidery arms and communicate with each other through electrochemical signals. Each of the brain's 100 billion neurons can connect to at least 1,000 other neurons, so a brain may have at least 100 trillion connections in all. Every second, a single neuron may send as many as 1,000 signals. These signals can zip from neuron to neuron at speeds up to 250 miles (400 kilometers) an hour.

Brain: Hub of the Nervous System

Neurons are the building blocks of the nervous system--a branching network of nerves that links all the parts of your body and connects you with the outside world. The brain is the hub of the nervous system. As you breathe, move, and interact with your surroundings, sensory nerves carry messages about what is happening throughout your body to your brain. Motor nerves relay signals from your brain to your muscles, telling you how to respond.

The spinal cord is the main highway of the nervous system. It carries signals from the body up to the brain, and from the brain out to the rest of the body.

Quick Facts

Sensory nerves carry messages about pressure, pain and temperature from the body to the brain.

Motor nerves carry messages from the brain to muscles and glands, telling them what to do.

Nerves on the right side of the body connect to the left side of the brain.

Nerves on the left side of the body connect to the right side of the brain.

Some messages shoot through the nervous system faster than others. When you pet a fluffy dog, one signal races from your fingertip to your brain in 10 to 20 milliseconds, letting you know she is soft. Another signal reaches your brain in a few milliseconds later, telling you she is warm.

The nervous system runs on electricity, but the levels are low. Brain signals involve less than one-tenth the voltage of an ordinary flashlight battery.

Brain Size: Comparatively Speaking

How big is the human brain compared to other brains?

Human brain: An adult human brain weighs around 3 pounds (1400 grams), or about as much as a cantaloupe.

Dog brain: The brain of a medium-sized dog weighs around 3 ounces (80 grams), the same as a small kiwifruit.

Grasshopper brain: A grasshopper brain weighs 1/1000 once (2 milligrams), or about as much as a grain of rice.